Three general types of reactors are used for biodiesel
production: batch reactors, semi-continuous-flow reactors, and continuous-flow
reactors.
The batch process is inexpensive, requiring much less
initial capital and infrastructure investment. It is flexible and allows the
user to accommodate variations in feedstock type, composition, and quantity.
The major drawbacks of the batch process include low productivity, larger
variation in product quality, and more intensive labor and energy requirements.
The semi-continuous process is similar to the batch
process except that the producer starts by reacting a smaller volume than the
vessel will hold and then continues to add ingredients until the vessel is
full. This process is labor intensive and not commonly used.
Continuous transesterification processes are preferred
over batch processes in large-capacity commercial production because these
processes result in consistent product quality and low capital and operating
costs per unit of product. The most common type of continuous-flow reactor is
the continuous stirred-tank reactor. Other types of continuous-flow reactors
are also used commercially, including ultrasonic reactors and supercritical
reactors. These alternative procedures can speed up the reaction.
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The batch reactor can be simply a tank that is
equipped with some type of agitation. The tank is filled with the reactants for
the process (in this case oil, alcohol, and catalyst), and then the agitator is
operated for some period of time. After the required time has elapsed, the
contents of the reactor are drained out and further processed.
The main characteristic of a batch reactor is that it
starts with unreacted material, causes it to react, and then at a later time
ends up with reacted material. That is, a batch reactor contains different
types of material depending on which time one happens to look at it.
Batch reactors are generally used in small biodiesel
production plants. A disadvantage of batch processes is that, to increase
production, it might be necessary to increase the physical size of the plant by
a proportional amount (by buying another reactor, for example). In contrast,
when using a continuous flow process, it is usually possible to increase the
plant’s production capacity by increasing the feed rate or shortening the
reaction time.
Continuous-Flow Reactors
The most common continuous-flow system in biodiesel
production is the continuous
stirred-tank reactor (CSTR). At first glance a CSTR, appears to
be identical to a batch reactor. In fact, often the actual reactor may be the
same, but additional controls are needed to set the reactor up in a
continuous-flow system. Some continuous-flow plants may be able to operate in
either batch or continuous mode.
In a CSTR, the reactants are continuously added and
the product (mixture of different chemicals, including unreacted reactants)
continuously withdrawn. Adequate agitation is required to ensure uniform
chemical composition and temperature. The continuous-flow process typically
requires intricate process controls and online monitoring of product quality.
When a CSTR is operated continuously at a steady
state, ideally the concentration of any chemical involved should be
approximately constant anywhere in the reactor and at all times. In reality,
this ideal state is rarely achieved; thus, adjustments need to be made to
operating parameters to ensure complete reaction.
Sometimes more than one reactor is used. In this
system, approximately 80 percent of the alcohol and catalyst are added to the
oil in a first-stage CSTR. Then, the reacted stream goes through a glycerol
removal step before entering a second CSTR. The remaining 20 percent of the
alcohol and catalyst are added to this reactor. This system provides a very
complete reaction with the potential of using less alcohol than single-step
systems.
Ultrasound is a useful tool to mix liquids that tend
to separate. In biodiesel production, adequate mixing is required to create
sufficient contact between the vegetable oil/animal fat and alcohol, especially
at the beginning of the reaction. Ultrasonic waves cause intense mixing so that
the reaction can proceed at a much faster rate.
Ultrasound transfers energy into fluid and creates
violent vibrations, which form cavitation bubbles. As the
bubbles burst, a sudden contraction of the fluid occurs, and the ingredients
are mixed in the area of the bubbles. Such a high-energy action in liquid can
considerably increase the reactivity of the reactant mixture and shorten the
reaction time without involving elevated temperatures. In fact, this reaction
can be achieved at or slightly above ambient temperature. Because there is no
need to heat the mixture, energy may be saved.
The ultrasound processing results in similar yields of
biodiesel with a much shortened reaction time compared to the conventional
stirred-tank procedure.
Ultrasound can be a good choice for small producers
(up to 2 million gallons per year capacity), who may only need one or two
ultrasound probes per reactor vessel. However, using ultrasound in large-scale
processing may be challenging because many ultrasound probes would be needed to
reach every area of the reactant mixture.
Traditional biodiesel production requires a catalyst
(usually sodium or potassium hydroxide) to complete the transesterification
reaction. After the reaction, the catalyst has to be removed to ensure fuel
quality. This can sometimes be problematic. To avoid the catalyst requirement,
transesterification can be achieved in a catalyst-free manner by using a
"supercritical" process.
A critical point of a fluid is defined by its critical
temperature and critical pressure, "the highest temperature and highest
pressure at which a pure chemical species is observed to exist in vapor/liquid
equilibrium.
At the supercritical
state, the phase boundary between liquid and vapor starts to
disappear, and the substance has qualities of both a liquid and a vapor.
When transesterification occurs during the
supercritical state of methanol (typically 300°C and 40 MPa/5800 psi or
higher), the vegetable oil or animal fat dissolves in methanol to form a single
phase. The reaction then occurs to reach completion in a few minutes without
any catalysts.
The supercritical process tolerates water and free
fatty acids in the system, and the soap formation that is common in the
traditional process is eliminated.
Since the supercritical state demands very high
temperature and pressure, the process can be expensive. Nevertheless, large
biodiesel producers may find this process to be cost effective because, since
the reaction happens so quickly, producers can make a large quantity with a
relatively small reactor and limited space.
Static mixers are
simple devices consisting of spiral-shaped internal parts within an enclosure,
such as a tube or pipe, that promote turbulent flow. They have no moving parts,
are easy to use and maintain, and are very effective at mixing liquids that are
not readily miscible under normal conditions.
Biodiesel production from vegetable oils and alcohols
is limited initially by the solubility of alcohol in vegetable oils. Static
mixers can be used to mix the reactants before they enter the reactor vessel. The
static mixer reactor is effective for biodiesel production. As with other
reactor configurations, temperature and catalyst concentration influence the
product yield significantly. The most favorable conditions for complete
transesterification are 60°C and 1.5% catalyst for 30 minutes. It is feasible,
therefore, to use a static mixer alone as the reactor for biodiesel preparation
from vegetable oils and alcohols.
A similar process is sometimes used commercially, but
the use of a large static mixer as the biodiesel processor has not been
commercialized.
Reactive
distillation (RD) is a chemical unit operation in which
chemical reactions and product separations occur simultaneously in one unit. It
is an effective alternative to the classic combination of reactor and
separation units.
Reactive distillation is a common chemical process in
situations where the reaction may reverse itself easily. The RD technique
removes the reaction products from the reaction zone, thus preventing the
reaction from reversing, and improving the overall conversion rate.
An RD system consists of numerous chambers with
openings from one to the next. Ingredients are added to the first chamber, and
as the mixture enters each successive chamber, the reaction progresses so that
by the last chamber, the reaction is completed. Both packed and tray columns
may be used for the RD applications; however, tray columns are preferred for
homogeneous reaction systems because of the greater liquid holdup and the
relatively longer retention time.
Reactive distillation systems have not been used
commercially in biodiesel production because RD tends to be a complex process.
However, the complexity is somewhat minimized when applied to biodiesel
production for a few reasons. The difference between the boiling temperatures
of methanol and fatty acid esters (biodiesel) is so large that the separation
of these two streams becomes very easy. Because the transesterification
reaction occurs in the liquid phase only, the reaction time is then established
by the total liquid holdup and the feeding rate of the reactants.
The RD reactor system has three major advantages over
the batch and traditional continuous-flow processes: 1) shorter reaction time
(10 to 15 min) and higher unit productivity (7 to 9 gallons per gallon reactor
volume per hour), which is highly desirable in commercial production units; 2)
much lower excess alcohol requirement (approximately 3.5:1 molar), which
greatly reduces the effort of downstream alcohol recovery and operating costs;
and 3) lower capital costs due to its smaller size and the reduced need for
alcohol recovery equipment.
